(a) Field of the Invention
The present invention relates to a transmission for a hybrid electric vehicle (HEV) having a high acceleration performance on start and in low speed ranges as well as having a simple structure of light-weight and small-size, resulting in minimizing energy loss.
(b) Description of the Related Art
Hybrid electric vehicles (HEVs) were conceived as a way to compensate for the shortfall in battery technology of an electric vehicle. So, HEVs combine the internal combustion engine of a conventional vehicle with the battery and electric motor of an electric vehicle.
Typically, the HEVs are classified into series, parallel, and split types according to a power transmission structure.
The series type HEV uses an engine just as a generator for generating electricity for a battery pack so as to overcome the drawback of the electric vehicle having a short driving distance. That is, the driving force of the series type HEV comes entirely from the electric motor as in a purely electric vehicle. The series type HEV has a disadvantage in that a power loss occurs during a electricity/power conversion and an AC/DC conversion. However, the engine never idles, which reduces vehicle emissions.
The parallel type HEV has a direct mechanical connection between the engine and the wheels as in a conventional vehicle, but also has an electric motor that drives the wheels so as to provide extra power to the driveline when power assist is needed for climbing on a slope or for quick acceleration.
The split type HEV is a compromise type having advantages of both the series and parallel type systems for optimizing energy efficiency. FIG. 2 schematically shows a split type HEV.
The split type HEV comprises a battery 112, an inverter 114 for inverting DC (direct current) from the battery 112 into AC (alternating current), a motor 116 for converting electrical energy from the inverter 114 into mechanical energy, a planetary gear set 120 acting as a mechanical energy distributor, and a final reduction gear 124 for transferring the driving torque to wheels.
The planetary gear set 120 includes a sun gear 128, a ring gear 130 connected to the final reduction gear 124, and a few pinion gears 132 connected by a carrier 134 and circumferentially interposed between the sun gear 128 and the ring gear.
In the split type HEV, the electric motor supplies power when the vehicle is starting and running at a low speed, and it can easily reverse the vehicle by changing a rotation direction of the motor 116. The vehicle uses the power created from the engine in a normal driving state and the motor acts as a generator for regenerating the battery during the engine operation. The motor can also provide extra power to the wheels when a power assist is needed for quick acceleration.
The series and parallel type HEVs use continuously variable transmissions (CVTs), and the adoption of the CVT increases the vehicle""s weight and causes energy loss through generating hydraulic pressure.
In the split type HEV, a planetary gear set interposed between the engine and the motor causes an increase in both the vehicle""s weight and manufacturing costs.
The present invention has been made in an effort to solve the above problems of the prior art.
It is an object of the present invention to provide a transmission for a HEV capable of improving acceleration performance on starting and in low speed ranges as well as minimizing energy loss with its simple structure of light weight and small size.
To achieve the above object, the present invention provides a transmission for a hybrid electric vehicle comprising a first motor connected to a battery through an inverter, the first motor functioning as a generator, a differential disposed between an engine and the first motor to provided driving force from one of the first motor and the engine, a final reduction gear engaged with the differential to transmit drive force to wheels, a second motor connected to the battery through the inverter to directly drive an axle shaft, a first one-way clutch disposed between the differential and the engine, and a second one-way clutch disposed within the differential.
Preferably, the differential comprises a first drive pinion connected to the engine by the first one-way clutch, a second drive pinion having a first end connected to the first drive pinion by the second one-way clutch and a second end connected to the first motor, first and second driven pinions driven by the first and second drive pinions, respectively, and a carrier engaged with the first and second driven pinions to transmit power of the first and second drive pinions to the final reduction gear.
Preferably, the first one way clutch is designed to transmit clockwise rotational force from the engine to the first drive pinion, and the second one-way clutch is designed to transmit clockwise rotational force from the second drive pinion to the first drive pinion.